With the advent of digital control technologies, aircraft system designers have a greater potential than ever before to link the aircraft and its engine to achieve revolutionary new capabilities for high performance aircraft. A critical aspect of aircraft systems design when utilizing jet power is the ability to throttle a turbojet engine very rapidly, while minimizing the risk of timeout.
Fuel control systems for a typical small expendable turbojet engine control fuel flow rate to control engine speed. The system controls speed based on an error signal calculated as a difference between desired and actual engine speed. With such a control system, a fuel servo valve must be custom calibrated for each engine to deliver the corresponding fuel flow for a given current command from the control system. Even after calibrating the fuel servo, the effect of fuel type and temperature and fuel flow rate must be compensated for when incorporating the fuel limits imposed by the control system. Certain consequences result from the above conditions. First, a custom servo calibration for each engine and its software becomes expensive. Secondly, after providing adequate safety margin into hard fuel limits over the operational range of the engine, the acceleration capability of the turbojet can be cut by more than eighty percent. If this safety margin is neglected, the engine risks a flameout at some operational point.
In addition to controlling engines with the above in mind, most fuel control systems use constant proportional and integral gains on the speed error in the formulation of the command to the fuel servo. In response to a stepped command to increase engine speed, the fuel servo position will change according to its time constant, and the gain settings of the speed controller. If a fuel limit is exceeded, unburned fuel will mix with burned products, producing a temperature drop across the turbine, preventing acceleration to its full potential. If fuel flow rate is increased further, the fuel/air ratio can exceed the rich limit capability of the combustor, and the flame is self extinguished. When a decrease in speed is commanded of the engine, the fuel servo responds in a similar manner as above for the positive step command. The major difference lies in the foresight of the controller being capable of recognizing a critical flame stability issue. When operating in the lean region, there is no warning that the flame will be lost. With small turbojet engines there is typically no restart capability.
The present invention is directed to solving one or more of the problems above.